1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on a recording material.
2. Description of the Related Art
Conventionally, in some image forming apparatus, an image is formed through use of electrophotography involving electrostatically controlling and attracting a developer (toner) formed of micropowder. The toner has property of being melted with heat, and hence it is necessary to take an increase in temperature in the image forming apparatus that is caused along with the operation of the image forming apparatus into consideration.
In particular, an apparatus main body has been downsized in recent years, resulting in a smaller relative distance between an image forming portion (process cartridge) containing a toner and electrical components such as a heat-fixing device, a motor, and an electric circuit board serving as heat-generation sources. Further, due to an increase in output speed of the image forming apparatus, rubbing between a photosensitive drum in the process cartridge and a drum cleaning blade or a developing roller has also become unignorable as the heat-generation sources. Therefore, it becomes still more essential to consider a change in temperature in the apparatus main body.
In order to solve the above-mentioned problem, the periphery of the process cartridge is kept at certain temperature or lower by introducing cooling air into the image forming apparatus through use of a cooling fan and appropriately arranging a duct serving as an air flow path.
Japanese Patent Application Laid-Open No. 2004-233452 proposes a configuration in which heat from a fixing portion is prevented from being transmitted to an image bearing member or an image forming portion.
Further, Japanese Patent Application Laid-Open No. 2005-326540 discloses a configuration in which, in order to keep, at a certain level or less, an increase in temperature in an image forming apparatus that is caused by the continuous operation of the image forming apparatus, for example, the heat generation amount is suppressed by increasing a sheet interval during continuous printing or switching the continuous operation to an intermittent operation involving suspension repeatedly to cool a heat-generation portion during suspension.
However, in recent years, there has been an increasing demand for lower noise in the image forming apparatus. Therefore, it is necessary to decrease the r.p.m. of the cooling fan. Thus, there arises a problem in that an air amount cannot be ensured sufficiently. Further, the downsizing of the apparatus main body and the increase in output speed have advanced. Therefore, it is also difficult to sufficiently ensure a clearance between the image forming portion and the heat-generation source and to decrease the throughput (processing performance per unit time) of a recording operation.
Further, in a color image forming apparatus, different printing speeds are set for color printing and monochrome printing in most cases. In particular, there is a demand for a further increase in output speed during monochrome printing. However, conventionally, common thermal design has been used for monochrome printing and color printing. Therefore, in some cases, cooling of the image forming apparatus during monochrome printing is not sufficient. In this case, under severe conditions such as image formation in a high-temperature environment and continuous image formation of forming a great amount of images at a time, the periphery of the process cartridge cannot be kept at predetermined temperature or lower in some cases.